Don't break hierarchy with filters for soft limits

Problem/Motivation

If you have a hierarchical facet combined to this module and soft limits for your facet, you will bump into this issue from Facets #3208580.
The selector are not optimized to collapse correctly if there is a hierarchy.

This module suffers from the same issue.

Steps to reproduce

* See #3208580
* enable facet search for the same facet,
* start typing: the list is filtered,
* remove all characters with the DELETE key,
* the list is broken and you can't use the soft links.

Proposed resolution

Apply a strict CSS selector to respect the hierarchy.